Fossil fuel combined cycle power system

ABSTRACT

A system for converting fuel energy to electricity includes a reformer for converting a higher molecular weight gas into at least one lower molecular weight gas, at least one turbine to produce electricity from expansion of at least one of the lower molecular weight gases, and at least one fuel cell. The system can further include at least one separation device for substantially dividing the lower molecular weight gases into at least two gas streams prior to the electrochemical oxidization step. A nuclear reactor can be used to supply at least a portion of the heat the required for the chemical conversion process.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The United States Government has rights in this invention pursuant toContract No. DE-AC05-00OR22725 between the United States Department ofEnergy and UT-Battelle, LLC.

FIELD OF THE INVENTION

This invention relates generally to a high efficiency fuel cell/gasturbine combined power generation system and a method of operating sucha system.

BACKGROUND OF THE INVENTION

New power systems operating on fossil fuels have been under developmentfor several years. These systems are designed to increase efficiency(fuel energy conversion to electricity) and to reduce harmful emissions(NO_(x), CO, CO₂) to the environment. Cogeneration and combined cyclesystem approaches can increase the efficiency by more than 20% comparedto conventional power systems.

Several cogeneration and combined-cycle power systems of variousconfigurations have been proposed that have the potential for achievingrelatively high efficiencies. However, these systems depend on obtainingsolutions to certain technical problems related to the concept. Forexample, these systems do not minimize harmful pollutants or maximizethermodynamic efficiency because they do not provide for recovery ofsynthesis from CO₂, and do not use fuel that passes through the fuelcells unreacted and do not efficiently use “waste heat” generated be thefuel cell stack.

Regarding thermodynamic inefficiency, the waste heat energy generated byfuel cells in these systems is used to drive closed water or open airpower cycles. Closed water or open air power cycles can bethermodynamically modeled as reversible heat cycles, if losses such asfrictional losses are ignored. For a reversible heat cycle whichoperates between two temperatures, maximum TH and minimum TC, themaximum cycle efficiency (e) is limited by the Carnot relation/equatione=1−(TC/TH), where both temperatures are expressed in units of Kelvin.

Thus, the maximum theoretical efficiency of a closed water or open airpower cycle is maximized when the cold reservoir is held as cold aspossible, and the hot reservoir is held as hot as possible.Consequently, since the range of attainable practical high and lowtemperatures are limited, the maximum possible efficiency derivable fromthese reversible heat cycles are lower than the Carnot limit. As aresult, practical efficiencies of these closed water or open air cyclescannot be higher than approximately 30 to 35%. Thus, the totalefficiency of the overall process of energy conversion to electricityfor an entire combined cycle cannot exceed approximately 55 to 60%. Tofurther maximize efficiency of combined cycle power systems which usefuel cells, a new combined cycle system is needed.

SUMMARY OF INVENTION

A method for converting fuel energy to electricity includes the steps ofconverting a higher molecular weight gas into at least one lowermolecular weight gas and supplying at least one of the lower molecularweight gases to at least one turbine to produce electricity. At leastone of the lower molecular weight gases is then electrochemicallyoxidized in fuel cells adapted to produce electricity from the lowermolecular weight gases. The method can further include the step ofsubstantially dividing the lower molecular weight gases into at leasttwo gas streams prior to the oxidizing step.

Separation devices can be used for the dividing step, preferably carbonfiber composite molecular sieves (CFCMS) or inorganic membranes. Each ofthe lower molecular weight gases can be electrochemically oxidized inthe fuel cells. The fuel cells can be solid oxide fuel cells (SOFC). Themethod can further include the step of directing at least a portion ofthe heat generated by the fuel cells for use in the conversion step.

A method for converting fuel energy to electricity includes the steps ofproviding a synthesis gas having a plurality of chemical components,substantially dividing the synthesis gas into at least two gas streamsand supplying at least one gas stream to a fuel cell to produceelectricity. The method can further include the step of driving at leastone turbine with at least one of the gas streams. The step of providingsynthesis gas can include a reforming step. In a preferred embodiment, agas principally containing methane (e.g. natural gas) is reformed in thereforming step, producing CO and H₂.

Separation devices can be used for the dividing step. The separationdevices can be carbon fiber composite molecular sieves (CFCMS) orinorganic membranes. The method can include the step of directing atleast a portion of heat generated by the at least one fuel cell to areformer.

The synthesis gas can include CO and H₂, wherein CO can be substantiallysupplied to a fuel cell adapted to electrochemically oxidize CO, and H₂can be substantially supplied to a fuel cell adapted toelectrochemically oxidize H₂. Preferably, the CO fuel cell and the H₂fuel cell are each solid oxide fuel cells. Carbon dioxide output by theCO fuel cell can be used to produce additional energy. The additionalenergy can be produced by using the CO₂ to drive a turbine. Outputstreams from at least one fuel cell can also be supplied to a combustionchamber for oxidation of fuel which may not have been fully oxidizedelectrochemically in the fuel cells.

Air supplied to the fuel cells can be first supplied to the CO fuel celland then to the H₂ fuel cell. The method can further include the step ofsupplying air to a device for providing oxygen enriched air to the fuelcells. The step of providing a synthesis gas can include reforming ahydrocarbon containing gas. The hydrocarbon containing gas canpreferably be methane or natural gas. The hydrocarbon containing gas canbe supplied to a reformer at a pressure of at least approximately 8atmospheres. In the preferred embodiment, the hydrocarbon pressuresupplied to the reformer is approximately at least 40 atmospheres, whichcorresponds to the gas pressure in a typical gas main. In an alternateembodiment of the invention, a portion of the output from at least onefuel cell is directed to a gas turbine.

A system for converting fuel energy to electricity includes a reformerfor converting a higher molecular weight gas into at least one lowermolecular weight gas, at least one turbine to produce electricity fromexpansion of at least one of the lower molecular weight gases, and atleast one fuel cell for electrochemically oxidizing at least one of thelower molecular weight gases to produce electricity. The system canfurther include at least one separation device for substantiallydividing the lower molecular weight gases into at least two gas streamsprior to the electrochemical oxidization step. The separation devicescan be carbon fiber composite molecular sieves (CFCMS) or inorganicmembranes.

Each of the lower molecular weight gases can be electrochemicallyoxidized in fuel cells. The fuel cells can be solid oxide fuel cells.The fuel cell may be a single fuel cell or multiple fuel cells in series(staged fuel cells). The system can further include a structure fordirecting at least a portion of heat generated by the fuel cells to areformer.

A system for converting fuel energy to electricity includes a device forproviding fuel having a plurality of chemical components, a separatordevice for substantially dividing the fuel into at least two gas streamsand at least one fuel cell adapted for electrochemically oxidizing thegas streams. The system can further include at least one turbine, whereexpansion of the fuel is used to drive the turbine. The device forproviding fuel to the system can be a reformer.

The reformer can reform a gas principally containing methane to produceCO and H₂. The separator device can be a carbon fiber compositemolecular sieve (CFCMS) or an inorganic membrane. A portion of the heatgenerated by the at least one fuel cell can be directed to the reformer.

In a preferred embodiment of the invention, the fuel mixture includes COand H₂. The CO can be substantially supplied to a fuel cell adapted toelectrochemically oxidize CO, and H₂ can be substantially supplied to afuel cell adapted to electrochemically oxidize H₂. The CO and H₂ fuelcells can be solid oxide fuel cells. Carbon dioxide output by the COfuel cell can be used to produce additional energy, preferably throughuse of a turbine.

The system can further include a combustion chamber, wherein outputstreams from at least one fuel cell can be supplied to the combustionchamber for oxidation of fuel which may not have been fully oxidized.Air is supplied to the fuel cells can be first being supplied to the COfuel cell and then to the H₂ fuel cell. The system can include a devicefor providing oxygen enriched air prior to delivery to the fuel cells.

When the system includes a reformer, the reformer can be used to converta hydrocarbon containing gas to fuel which can be separated andelectrochemically oxidized. The hydrocarbon containing gas canpreferably be selected from a mixture principally being methane gas ornatural gas. The natural gas is preferably supplied to the reformer at apressure of at least approximately 8 atmospheres. More preferably, thenatural gas pressure is supplied to the reformer from a gas main withpressure of at least 40 atmospheres, which allows for efficient use ofthe pressure in a gas main to obtain additional energy in the system.

BRIEF DESCRIPTION OF THE DRAWINGS

A fuller understanding of the present invention and the features andbenefits thereof will be accomplished upon review of the followingdetailed description together with the accompanying drawings, in which:

FIG. 1 illustrates a schematic of a basic combined power systemconfiguration in accordance with an embodiment of the invention.

FIG. 2 illustrates a schematic of a modified combined power systemconfiguration in accordance with an embodiment of the invention.

FIG. 3 illustrates a schematic of a modified combined power systemconfiguration in accordance with another embodiment of the invention.

FIG. 4 illustrates a schematic of a modified combined power systemconfiguration in accordance with yet another embodiment of theinvention.

FIG. 5 illustrates a schematic of a modified combined power systemconfiguration in accordance with yet another embodiment of theinvention.

FIG. 6 illustrates a schematic of a modified combined power systemconfiguration in accordance with yet another embodiment of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An apparatus and method for producing a high efficiency electrical poweroutput combines fuel cells and gas turbines. The invention can increasethe utilization efficiency of fuels, such as natural gas, in the processof fuel energy conversion to electricity by approximately 20 to 30%compared with existing power systems to approximately 80 to 85%. Inaddition, the invention produces substantially fewer environmentallyharmful emissions compared to other power systems, generating up toapproximately three times less harmful emissions compared to existingpower systems. Another advantage of the invention is the production ofsignificant quantities of drinking water.

The apparatus and diagram showing the basic configuration of anembodiment of the invention is shown in FIG. 1. Although specificchemicals, operating conditions and system interconnections are showntherein, the invention is in no way limited to the specific chemicals,operating conditions and system interconnections which are shown in FIG.1.

Combined power system 100 includes a fuel source 101, such as methane ornatural gas, which enters the reformer 102 at a pressure above ambientpressure. As used herein, natural gas refers to a mixture of gases thatprincipally includes methane together with varying quantities of ethane,propane, butane, and other gases. Preferably, the fuel source pressureprovided is at least 40 atmospheres, which corresponds to the pressurein a typical gas main. The pressurized fuel is preferably fed to thereformer 102 through a heat exchanger 103 to heat the fuel prior todelivery to reformer 102. Similarly, steam is preferably fed to reformer102 by passing water through one or more heat exchangers, such as 104and 105. Heat exchanger 104 is preferably used for heating water whileheat exchanger 105 is preferably used for turning the water heated byheat exchanger 104 into steam. Given their differing purposes, heatexchangers 104 and 105 will preferably each feature designs appropriatefor their specific purposes.

In the case of methane fuel, the reforming process results in theformation of a synthesis gas having CO and H₂:CH₄+H₂O=CO+3H₂

Hot synthesis gas can also be produced by natural gas reforming, partialoxidation, or alternatively by coal gasification or supplied from anexternal source. As used herein, synthesis gas is a mixture of gaseswhich can be used as a feedstock for a chemical reaction. For example,carbon monoxide and hydrogen to make hydrocarbons or organic chemicals,or hydrogen and nitrogen to make ammonia are considered synthesis gases.

However, appropriate fuel for use in system 100 includes generally anygas which can be converted (e.g. reformed) into one or more lowermolecular weight components, at least one of the lower molecular weightcomponents being electrochemically oxidizable. Hereinafter, the term“synthesis gas” will refer to one or more lower molecular weightcomponents derived from a higher molecular weight compound, provided atleast one of the lower molecular weight components is capable of beingelectrochemically oxidized. Although specific examples and systemdescriptions to follow refer to a synthesis gas which contains H₂ and COand appropriate apparatus to efficiently process these gases, theinvention is in no way limited to use of H₂ and CO and the associatedapparatus shown and described herein.

Synthesis gas output from reformer 102 is directed to turbine 120,preferably at a pressure of at least 40 atmospheres, which correspondsto the pressure in a typical gas main and at high temperature (e.g. 1000to 1100° K), where it can be expanded to produce electricity.Alternatively, synthesis gas can be supplied externally, removing theneed for reformer 102. In any event, an air compressor 106 and anelectric generator 107 may also be driven by the energy produced by theexpansion of the synthesis gas.

The thermodynamic efficiency of the turbine expansion process isincreased compared to prior systems in at least two ways. The use of aworking fluid or working fluid mixture having a high specific volume(such as CO and H₂) to power turbine 120 results in an increased powerdensity and energy conversion efficiency for the overall power systemcompared to systems which use lower specific volume working fluids, suchas conventional combustion products (e.g. CO₂ and air). The relationshipbetween turbine work and a working fluid's specific volume is thefollowing:

$\frac{\mathbb{d}H}{\mathbb{d}P} = {{V\mspace{14mu}{or}\mspace{14mu}\frac{\Delta\; H}{\Delta\; P}} = V_{av}}$where ΔH is the change of enthalpy in the turbine which is equivalent tothe work produced; ΔP is the change of pressure in the turbine andV_(av) is the average specific volume of the working fluid in theturbine. Based on the above relation, assuming the same change inpressure (ΔP), the work produced by the turbine from expansion of theworking fluid is proportional to the average specific volume of theworking fluid. Thus, more work can be produced by turbine expansionthrough use of higher average specific volume working fluids.

In a preferred embodiment of the invention, the working fluid suppliedto turbine 120 is synthesis gas comprising CO and H₂. Its specificvolume (equal to its; volume divided by its mass) is approximately twotimes greater than that of steam and three times greater than that ofair, all other conditions being equal. Thus, assuming the same rate ofexpansion in turbine 120, the specific power (power/mass) generated bythe expansion of the synthesis gas is approximately two and three timesgreater, respectively, compared to turbines which use steam or air asthe working fluid. High specific power densities produced by theivvention permit turbine 120 to have lighter weight and smallerdimensions. As a result, the use of synthesis gas for turbine expansionprovides lower overall system cost compared to other power systems. Inaddition, the use of synthesis gas as the working fluid for turbineexpansion largely avoids the inherent thermodynamic efficiencylimitations imposed by the Carnot principle on conventional powersystems which use cyclic processes because the synthesis gas used bysystem 100 goes to the turbine 120 at an elevated pressure (e.g thepressure of a typical gas main) and is subsequently reactedelectrochemically.

After expansion in the turbine 120, hot, reduced-pressure synthesis gascan be directed to heat exchangers, such as 104 and 105, where the hotsynthesis gas can release heat. The heat released can be used to producesteam. Cooled synthesis gas can then be directed to separation device115 for substantially splitting the synthesis gas (e.g. H₂ and CO)substantially into its component flow streams. For example, in the caseof methane supplied to a reformer, separation device 115 allowsseparation of the mixed CO and H₂ gas stream substantially into itscomponents, CO and H₂. Preferably, gas separation device 115 is aninorganic membrane type separator and/or a carbon fiber compositemolecular sieve (CFCMS). CFCMSs feature two-mode operation, havingdistinct adsorption and desorption cycles. Accordingly, in theembodiment of the invention which uses CFCMSs, system 100 utilizes atleast two (2) CFCMSs connected in parallel, and phased appropriately tosupport a continuous output.

The degree of component separation attainable from a given separationdevice 115 depends on the separator design characteristics, flowthermodynamic parameters and gas occupation time. For example, a systemsuch as system 100 shown in FIG. 1 having CFCMS membrane separatorsseparates H₂ and CO under a pressure drop of approximately 6 atm. Underthese conditions, the resulting separation isolates approximately 80–85%of all H₂ from the mixed synthesis gas stream using a CFCMS separator ofappropriate dimensions.

Incomplete H₂ separation does not significantly influence the efficiencyof system 100. Hydrogen left in mixture with the CO after separation isprovided to the CO fuel cell 110 where it can be oxidizedelectro-chemically together with CO to produce electricity.

Assuming use of a synthesis gas having CO and H₂, following separationby separator 115, the gas stream containing the H₂ flow can bepreferably be directed to a hydrogen fuel cell 111, while the CO flowcan be preferably directed to a separate carbon monoxide fuel cell 110.The fuel cells may be a single fuel cell or multiple fuel cellsconnected in series (staged fuel cells). Both fuel cells generate powerthrough oxidation of the H₂ and CO provided, forming water and carbondioxide, respectively. Since both CO and H₂ can be electrochemicallyoxidized by fuel cells, the invention, the combined cycle system canproduce an efficiency of up to 40% resulting from solely the direct fuelcell 110 and 111 conversion of synthesis gas chemical energy toelectricity.

In addition to electricity produced, both fuel cells 110 and 111generate significant quantities of heat from the respectiveelectrochemical oxidation processes. The overall system efficiency canbe substantially increased through efficient utilization of the “wasteheat” generated by the fuel cells for cogeneration (combined heat andpower). In the preferred embodiment of the invention, the combinedsystem cycle uses heat generated by the fuel cells to supply heat toreformer 102 and heat exchanger and/or supply heat to power a turbine,such as turbine 120, to produce an additional source of electricalpower.

The mixture of steam (and nitrogen, assuming air is used) output by fuelcell 111 at high temperature is preferably directed to reformer 102,where it can provide heat for the reforming process, and then can bedirected to heat exchangers 104 and 105 where the mixture can releasemost of its remaining heat. The cooled steam (and nitrogen, assuming airis used) can then be directed to a condenser-separator 112 where watercan be condensed and nitrogen can be returned to the atmosphere. Heatproduced by the condensation process can be used, for example, topreheat air from compressor 106.

Similarly, hot CO₂ exhaust from fuel cell 110 is also preferablydirected to reformer 102. After reformer 102, exhaust gases from fuelcell 110 can be directed to heat exchangers, such as heat exchangers 113and 103, to release most of its remaining heat. The cooled CO₂ gas canthen be directed to a membrane or CFCMS separator 117 where the CO₂ canbe separated from nitrogen (if air is used as the oxygen containing gasfor fuel cell electrochemical oxidation), the nitrogen released to theatmosphere while the CO₂ can be preferably released to sequestration.

In the preferred embodiment of the invention, the fuel cells 110 and 111used are solid oxide fuel cells. Solid oxide fuel cells are essentiallyall-ceramic power generating devices which use air (or oxygen) and fuelflows to generate electricity and heat. Thus, like a conventional fuelcell, they produce electric power by an electrochemical reaction,avoiding the air pollutants and efficiency losses associated withtraditional combustion processes. For example, zirconia electrolytes canbe used to allow the cells to operate at higher temperatures than otherfuel cells, producing more energy per unit of fuel and substantiallyless carbon dioxide (a greenhouse gas). Solid oxide fuel cells do notuse boiling liquids or moving parts to generate electricity.Accordingly, solid oxide modules can be expected to operate reliably formany years.

Fuel cells, such as solid oxide fuel cells, provide simple outputadjustment. Thus, power systems according to the invention can alsoprovide the capability to adapt quickly to changes of external loadwithout a significant decrease in efficiency. Through the convenientadjustment of air (or oxygen) and fuel flows, fuel cells can be easilyadjusted for changing demands for electricity by boosting output whennecessary, then cycling down output when demand becomes reduced.

Fuel cell materials and designs have resulted in the development ofsolid oxide fuel cell configurations with the capability of achievingvery high fuel utilization rates. For example, up to 90% or more of thefuel fed to solid oxide fuel cell stacks can be utilized. However, otherfuel cell types, such as molten carbonate, alkaline, PEM or phosphoricacid fuel cells can also be used with fuels such as H₂ in the invention.

The oxygen required for fuel cell operation can be supplied from anoxygen containing gas, such as air, and preferably provided by acompressor 106. Alternatively, oxygen-enriched air can be provided.Oxygen enrichment can be achieved through use of a separator device,such as a CFCMS. As a further alternative, substantially pure oxygen canbe supplied to the fuel cells 110 and 111. The oxygen containing gas canbe preferably preheated by a condenser-separator 112 and thenadditionally heated by heat exchanger 113 before being supplied to fuelcells 110 and 111.

Even though system 100 uses water in the reforming process, system 100does not require an external source of water because the system 100 is anet water generator. For example, a 1 MW system can produceapproximately 10 tons of water each day. The water produced by thesystem can be used for a variety of purposes. Approximately one third ofthe water formed in the cycle can preferably be fed to the heatexchanger 109 under high pressure with the help of a pump 114 beforebeing supplied to reformer 102. Approximately two-thirds of the waterformed can preferably be provided to consumers or may be safelydiscarded because it is environmentally safe.

Depending on the purpose and conditions of application, the proposedbasic system configuration can be modified to achieve certain improvedcharacteristics. FIG. 2 illustrates a schematic of a modified combinedpower system 200 configuration in accordance with an embodiment of theinvention which can be used to achieve higher efficiencies. Thisembodiment can also result in deeper cleaning of exhausted gases fromCO, CO₂ and NO_(X), compared to the basic system configurationillustrated in FIG. 1.

Referring to FIG. 2, following expansion through turbine 120, thesynthesis gas can be provided to the separator device 115 at a higherpressure compared to the embodiment shown in FIG. 1. For example, thiscan be accomplished by sacrificing some turbine expansion, resulting ina synthesis gas pressure after turbine 120 being higher than the basicembodiment shown in FIG. 1. Higher pressure at separator 115 permits aconsiderable reduction in the required separator 115 dimensions, whilestill providing for the separation of approximately 85 to 90% of H₂ fromthe synthesis gas mixture. With this configuration, the H₂ fuel cell 111will operate at a pressure of approximately 1 atm., and the CO fuel cell110 will operate at a pressure of approximately 6 atm.

In this embodiment, air (or oxygen-enriched air or oxygen) flows fed tothe respective fuel cells are fed from different stages of thecompressor 106. Fuel cell 100 receives air (or oxygen-enriched air oroxygen) directly from compressor 106, while fuel cell 111 receives air(or oxygen-enriched air or oxygen) after passing throughcondenser-separator 112 and heat exchanger 113. This configurationadvantageously provides additional output power derived from the powerof the CO₂ flow after fuel cell 110. A turbine 119, which can driveelectric generator 118, can also be included in the system 200.

Two separate flows leave the fuel cell 110. The first flow contains theproducts of oxidizing CO to CO₂ and part of the air flow used as anoxidizer, and the second flow contains the rest of the fuel cell 110output flow. The first flow can go to turbine 119 under a pressure ofapproximately 6 atm, and then can be directed to heat the working fluidin reformer 102. The first flow can then be directed to heat exchangers113 and 103.

The second flow is mixed with the output flow from fuel cell 111 thatcontains water, unreacted H₂ and air having increased N₂ content. Theoxygen in the mixture reacts with unreacted hydrogen resulting in nearlyfull oxidization. Heat released from the oxidation can be used forheating the working fluid in reformer 102.

The first output flow fuel cell 110 is moved separately from the outputstream from fuel cell 111. This permits separator 117 to efficientlyseparate out N₂ from the system for atmospheric release and facilitatethe capture of CO₂ for sequestration.

FIG. 3 illustrates a schematic of a modified combined power system 300configuration in accordance with another embodiment of the inventionthat can be used to reduce the environmental impact of the system. Onemethod of reducing environmental impact is through increasing the systemefficiency. To utilize fuel more completely, combustion chambers 121 and122 can be provided to receive the electrochemical oxidation productsand non-utilized fuel output by the fuel cells 110 and 111,respectively. Combustion chambers 121 and 122 can provide a secondchance to extract energy from unreacted fuel by more fully oxidizingnon-utilized fuel output by the fuel cells to produce additional heatenergy which can be converted into additional electricity.

In this embodiment, air from compressor 106 at pressure of approximately6 atm can be directed first to the CO fuel cell 110 and then to the H₂fuel cell 111. This can result in better utilization of oxygen in theair. Alternatively, an oxygen separation membrane (not shown) can beused to supply higher oxygen concentrations (rather than air) to one orboth of the fuel cells 110 and 111. This results in better utilizationof oxygen in the air. Alternatively, oxygen-enriched air or oxygen mayprovided from external sources (not shown). In that event, airseparation device based on CFCMSs or solid state ceramic membranes canbe used (not shown).

The system shown in FIG. 3 also divides heat output by CO fuel cell 110between reformer 102 and turbine 119. Turbine 119 receives the outputflow from CO fuel cell 110 at nearly 6 atm. since reformer 102 and heatexchanger 103 result in little reduction in pressure of the output flow.However, reformer 102 and heat exchanger 103 extract significant heatfrom CO fuel cell 110 output flow. Accordingly, turbine 119 is primarilypowered by the remaining pressure of the CO fuel cell 110 output flowwhich reaches turbine 119.

FIG. 4 illustrates a schematic of a modified combined power systemconfiguration in accordance with yet another embodiment of the inventionwhich can be used to generate higher output power without increasing thesize of system components. By placing the gas turbine 120 before thereformer 102 in the power cycle, rather than after the reformer 102,higher power results by virtually eliminating synthesis gas leakagethrough the turbine shell. This leakage can significantly reduce systemoutput power. However, with this arrangement, the efficiency of theoverall system may decrease a small amount. As in FIG. 3, system 400also includes combustion chambers 121 and 122 to receive theelectrochemical oxidation products and non-utilized fuel output by thefuel cells 110 and 111, respectively, to utilize fuel more completely.

Use of pure oxygen can increase fuel cell 110 and 111 output power up byapproximately 20% compared to fuel cells which use air to provideoxygen. Alternatively, oxygen rich air may be used by adding separator123. In that event, separator 123 can preferably be CFCMS or solid stateceramic membranes.

The systems shown in FIGS. 2–4 each divide the heat evolved by fuel cell110 between the reformer 102 and the turbine 119. With this arrangement,the Carnot cycle limitation is further weakened as the flow containingCO was not compressed in the compressor before. In principle, theefficiency of any use of a fuel cell's heat in a Brayton and/or Rankinecycle will be limited by the Carnot cycle efficiency.

FIG. 5 illustrates a schematic configuration of a modified combinedpower system in accordance with yet another embodiment of the inventionwhich provides at least a portion of the heat required for the reformingprocess by a nuclear reactor 124. In this regard, nuclear reactor 124may be a fission or fusion type reactor. Methane, natural gas or asimilar fuel or mixture of fuels, is first heated by nuclear reactor124. Heated fuel can then, at least in part, flow to a gas turbine 120,where it can be expanded to produce electrical power; and then bedirected to reformer 102. A portion of the fuel output by nuclearreactor 124 can be directly provided to reformer 102. As shown by thedashed line from nuclear reactor 124 and reformer 102, water (steam) canalso be heated by nuclear reactor 124 before being supplied to reformer102.

Flows of the products of electrochemical oxidation from fuel cells 110and 111 can be directed to additional combustion chambers 121 and 122and then to gas turbines 125 and 119, where they can produce additionalelectrical power by driving electric generators 126 and 118,respectively. The power system shown in FIG. 5 has an importantadvantage over conventional nuclear plants as it is allows for theutilization of nuclear reactor generated heat with an efficiency ofapproximately 80–85%, compared to an efficiency of approximately 27–30%achieved by conventional steam-turbine nuclear plants. The modificationproposed as shown in FIG. 5 can be applied both to new combinednuclear/fossil fuel power plants and to redesigned operating nuclearpower plants so that they may become more energy efficient.

FIG. 6 illustrates a schematic configuration of a modified combinedpower system in accordance with yet another embodiment of the inventionwhich also provides nuclear reactor 124 as in FIG. 5, but directssynthesis gas output by reformer 102 to turbine 120 prior to thesynthesis gas being directed to separator 115. Energy produced byexpansion of the synthesis gas in turbine 120 can be used to drive aircompressor 106 and electric generator 107. This configuration may becontrasted to the embodiment shown in FIG. 5, where synthesis gas outputby reformer 102 is fed directly to separator 115. This configuration canprovide an increase in system output power, compared with the outputpower available from the system shown in FIG. 5.

While the preferred embodiments of the invention have been illustratedand described, it will be clear that the invention is not so limited.Numerous modifications, changes, variations, substitutions andequivalents will occur to those skilled in the art without departingfrom the spirit and scope of the present invention as described in theclaims.

1. A system for converting fuel energy to electricity, comprising: areformer for converting a higher molecular weight gas into at least onemixed gas stream of lower average molecular weight comprising at least afirst lower molecular weight gas and a second gas, said first and secondgases being different gases, wherein said first lower molecular weightas comprises H₂and said second gas comprises CO; at least one turbinecoupled to an electrical generator having an input connected to anoutput of said reformer, said turbine receiving said mixed gas streamand generating electricity from expansion of said mixed gas stream; aseparator connected to an output of said turbine, said separator havinga first and a second output for dividing said mixed gas stream, whereina first gas stream mainly comprising said H₂ is provided at said firstoutput and a second gas stream mainly comprising said CO is provided atsaid second output; a first fuel cell, an anode of said first fuel cellconnected to said first output for electrochemically oxidizing saidfirst gas stream to produce electricity; and a second fuel cell, ananode of said second fuel cell connected to said second output forelectrochemically oxidizing said second gas stream to produceelectricity.
 2. The system for converting fuel energy to electricity ofclaim 1, wherein said separator comprises at least one selected from thegroup consisting of carbon fiber composite molecular sieves (CFCMS) andinorganic membranes.
 3. The system for converting fuel energy toelectricity of claim 1, wherein said fuel cells are both solid oxidefuel cells.
 4. The system for converting fuel energy to electricity ofclaim 1, further comprising structure for directing at least a portionof heat generated by said fuel cells to said reformer.
 5. The system forconverting fuel energy to electricity of claim 1, further comprising anuclear reactor for generating heat.
 6. The system for converting fuelenergy to electricity of claim 5, wherein at least a portion of saidheat is directed to said reformer to heat said higher molecular weightgas.
 7. The system for converting fuel energy to electricity of claim 1,wherein said higher molecular weight gas is provided by a pipeline whichprovides pressurized natural gas, said mixed stream being directlyprovided to said turbine from said reformer without additional steps foreither compressing or heating said mixed gas stream.
 8. The system forconverting fuel energy to electricity of claim 1, wherein said highermolecular weight gas principally contains methane and is reformed bysaid reformer, wherein said first lower molecular weight gas comprisesH₂ and said second gas comprises CO.
 9. The system for converting fuelenergy to electricity of claim 1, wherein at least one of said first andsecond fuel cells produces a CO₂ output, wherein expansion of said CO₂is used to produce additional energy.
 10. The system for converting fuelenergy to electricity of claim 1, further comprising a combustionchamber, wherein output streams from at least one of said fuel cells aresupplied to said combustion chamber for oxidation of fuel which has notbeen fully oxidized.
 11. The system for converting fuel energy toelectricity of claim 1, wherein said first and second fuel cellscomprise H₂ and CO fuel cells, respectively, and air is supplied to bothsaid first and second fuel cells, said air first being supplied to saidCO fuel cell and then to said H₂ fuel cell.
 12. The system forconverting fuel energy to electricity of claim 11, wherein said air issupplied to a device for providing oxygen enriched air prior to deliveryto said fuel cells.
 13. The system for converting fuel energy toelectricity of claim 1, wherein said hydrocarbon containing gas isnatural gas, said natural gas being supplied to said reformer at apressure of at least approximately 40 atmospheres.